There's been a bunch of debate as to whether iTunes' uniform-pricing scheme makes sense for music, and basically the same arguments would fit here. I've discussed this before. It seems to make sense only as a marketing ploy; other reasons that might be used to support uniform-pricing don't seem to hold water. The other point worth noting is that the argument between the MPAA and Apple is not about $9.99/each versus $5-$15/each, but rather $9.99/each versus $10-$20/each. So it's not just a disagreement about the tiering of pricing, but also the level of pricing.
Of course, another form of movie sales uses uniform pricing: theater-going. This is also a peculiar instiution; Barak Orbach and Liran Einav take a stab at explaining that pricing scheme as a result of the "peculiar veritical integration" that exists in film distribution.